Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been demonstrated in an extended-cavity semiconductor laser by using a diffraction grating filter design. Obtaining single-frequency laser operation and ensuring mode-hop-free tuning, however, may use a complicated mechanical apparatus and limit the maximum tuning speed. One of the fastest tuning speeds demonstrated so far has been limited less than 100 nm/s. In certain applications such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width may provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy.
A line width on the order of 10 GHz is readily achievable with the use of an intra-cavity tuning element (such as an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously demonstrated has been less than 1 kHz limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 15 kHz may be needed for video-rate (>30 frames/s), high-resolution optical imaging in biomedical applications.
Accordingly, there is a need to overcome the above-described deficiencies.